When making measurements in radio frequency (RF) range, a set of signals at equally spaced frequency intervals (referred to as a “comb spectrum”) are often analyzed. For example, a network analyzer may excite a device under test (DUT) using a comb spectrum and then analyze the DUT's response to the comb spectrum. The DUT's response is typically another comb spectrum in which individual frequency components of the original comb spectrum are attenuated, amplified, and/or phase shifted according to the DUT's electrical characteristics.
To analyze the DUT's response, it may be necessary to downconvert the comb spectrum to a lower frequency range compatible with signal analysis circuitry. For instance, when analyzing a DUT using a comb spectrum in a higher frequency range (e.g., comparatively high gigahertz frequency range), it may be necessary to downconvert the comb spectrum to a lower frequency range (e.g., a lower gigahertz frequency range or a megahertz frequency range). This downconversion can be performed, for example, using a superheterodyne receiver.
FIG. 1 shows a known system that excites a DUT using an input comb spectrum (or test comb spectrum) and then downconverts a resulting output comb spectrum (or response comb spectrum) for analysis.
Referring to FIG. 1, a system 100 comprises a network analyzer 105, a DUT 110, and a downconverter 115. Network analyzer 105 uses a comb synthesizer to generate a comparatively high frequency input comb spectrum for DUT 110. DUT 110 receives the input comb spectrum and responds by producing a comparatively high frequency output comb spectrum. Downconverter 115 downconverts the output comb spectrum from the comparatively high frequency to a lower, intermediate frequency (IF) compatible with signal analysis circuitry in network analyzer 105, and transmits the downconverted output comb spectrum to the signal analysis circuitry.
In certain known test systems, the downconversion of a comb spectrum may also produce problems resulting in ambiguities in the downconverted comb spectrum. These problems include so-called “image response” and so-called “harmonic mixing”. Image response occurs when two frequency components in the input comb spectrum are equidistant from a frequency of a local oscillator (LO) signal, so they map to a single frequency component in the output comb spectrum. Harmonic mixing, on the other hand, occurs when two frequency components in the input comb spectrum mix with two different harmonic components of the LO signal such that they map to the same frequency component in an output comb spectrum.
FIG. 2 shows an example of image response. In FIG. 2, a mixer 200 mixes an LO signal 210 having a frequency of 10 MHz with an input comb spectrum 205 having frequency components at 9 MHz, 10 MHz, and 11 MHz. This mixing produces an output comb spectrum 215. As illustrated by a dotted arrow in FIG. 3, the 9 MHz component and the 11 MHz component of input comb spectrum 205 both mix with the 10 MHz LO signal 210 to produce a component at 1 MHz in output comb spectrum 215. In other words, the 9 MHz and 11 MHz components both map to the same frequency component, as indicated by a dotted arrow. This creates an ambiguity in output comb spectrum 215, making it difficult if not impossible to distinguish the downconverted 9 MHz component from the downconverted 11 MHz component.
Because the 9 MHz signal is lower than the 10 MHz LO signal, the 9 MHz signal is referred to as a “lower sideband” signal and the corresponding mixing product of the 9 MHz signal is referred to as a “lower sideband” response. Similarly, because the 11 MHz signal is higher than the 10 MHz LO signal, the 11 MHz signal is referred to as an “upper sideband” signal and the corresponding mixing product of the 11 MHz signal is referred to as an “upper sideband” response.
FIG. 3 shows an example of harmonic mixing. In FIG. 3, a mixer 300 mixes an LO signal 310 having a frequency of 10 MHz with an input comb spectrum 305 having frequency components at 9 MHz and 31 MHz. This results in an output comb spectrum 315.
In addition, mixer 300 has a harmonic response, meaning that the mixer 300 mixes the harmonics of LO signal 310 with comb spectrum 305. In FIG. 3, the second and third harmonics of LO signal 310 are shown in parentheses to distinguish them from LO signal 310, which is used to drive mixer 300. In this example, the 9 MHz input signal mixes with the 10 MHz LO signal to produce an output component at 1 MHz, and the 31 MHz input signal mixes with the 30 MHz harmonic of LO signal 310 to produce an output component at 1 MHz as well, as indicated by a dotted, arrow. Accordingly, the 9 MHz and 31 MHz components both map to the same frequency component, creating an ambiguity in output comb spectrum 315.
Although FIGS. 2 and 3 illustrate the problems of image response and harmonic mixing with discrete frequency components, these problems also arise in superheterodyne receivers that use a “block downconverter” configuration. In superheterodyne receivers, a band of input frequencies is converted simultaneously as a block to a comparatively lower band of frequencies. For example, an input band of 8 MHZ to 9 MHz could be mixed with 10 MHz signal to yield an output (IF) band of 1 MHz to 2 MHz.
In conventional superheterodyne receivers, the problems of image response and harmonic mixing are typically addressed, through the use of a preselector, which is a tunable filter that passes one band of frequencies at a time to be received, The preselector may comprise, for instance, a bandpass or notch filter that eliminates unwanted frequencies from the input signal. This prevents two input components from simultaneously mapping to the same output component, so spurious components can be removed from a DUT response.
Unfortunately, a preselector is typically a fairly expensive, complex subassembly that cannot be implemented on an integrated circuit (IC). For example, an Yttrium-Iron-Garnet (YIG) filter is often used, at substantial cost and complexity.
In view of the foregoing, there is a need for superheterodyne receivers that can achieve reliable performance without the use of preselectors.